[llvm] [RISCV] Check only demanded VTYPE fields in needVSETVLIPHI (PR #90168)

[Luke Lau via llvm-commits]

lukel97 wrote:

Closing as I think there should be a better way to avoid the saxpy_vec regression in #89089. I'm not sure what it is yet though

https://github.com/llvm/llvm-project/pull/90168